Monday, July 13, 2009

Choosing Spring Loaded Devices for Positioning and Locking

Diane here, to talk to you today about the engineer’s options when he/she needs to align, hold, or latch different parts of equipment together. When this is your objective, you need a spring loaded device, and the options for that component are many and varied.

Breaking it down most simply, these sorts of devices consist of a body, an internal spring, and a ball or nose. They may also have a knob, button, or lever for the operator to control by hand. The general purpose of these components is to use the force in the spring to apply pressure so that the ball or nose will remain nested in some sort of hole or other receptacle. This way two or more elements of the equipment can be aligned or fixed into a determined relative position.

There are countless uses for positioning and locking components. They are used in workholding applications, as well as for indexing, latching, ejecting, and even electrical contacts.

So let’s examine the various types of components in this category (featured in our Section 10, Indexing, Spring and Ball Plungers).

Indexing plungers: These have a beveled or rounded nose that inserts easily into an indexing receptacle. The nose is long enough to fix the position firmly, preventing movement in any direction. In order to move the position of the plunger, the operator must mechanically retract the nose. An example is this GN 617 indexing plunger (non-lock out type).

Spring plungers: Spring plungers differ from indexing plungers in that they have a nose designed to allow for movement when some side force is applied. In other words, it isn’t necessary (or possible) to retract the nose mechanically. Consequently, these plungers are not appropriate if heavy side load is involved in the application. Shown is our SPNL short spring plunger (note the nylon locking element in the side of the threaded body).

Hand-retractable spring plungers: These plungers are sort of a hybrid of the two above types, and include a knob for manual retraction of the plunger. Among other things, they are used for manual or automated holding, as in workholding applications. Here you see our LRSS stainless steel hand retractable spring plunger (lock out type).

Ball plungers: These have a ball instead of a nose, and no knob. The shallow depth of the ball allows for easy movement in and out of position, but as with spring plungers, they do not perform well with heavy loads. The illustration is our GN 615 steel ball plunger with threaded body (note the slot provided for installation with a screwdriver).

Side thrust pins: These components are designed for fixturing small parts and holding them in place with constant pressure, as shown in the illustration. In the photo you see the GN 713 zinc-plated steel side thrust pin, which has a threaded body.

Quick release pins: Instead of body with a nose, these components consist of a rod or stem, from which small side securing pins protrude. A button in the handle retracts or extends these pins so that the quick release pin can be removed or secured in place (see application example).

The component in the illustration is our GN 114.2 steel rapid release pin. We offer many other options for quick release pins, including stainless steel components, heavy duty construction, and self-locking features.

So far this seems fairly simple, right? Spring-loaded components aren’t rocket science (especially if yours truly can explain them). We offer these types of components in many sizes, both inch and metric. But there are more options than that to consider. Let’s start with balls and noses.

Balls and noses: Balls, as mentioned above, allow for easy position changes. However, their holding force is limited. Noses may be round for reduced friction and similar easy positioning. They can also be chamfered for easier insertion into indexing holes. Flat noses have a larger contact area but will obviously resist side motion. Hex noses can allow installation of the plunger from the front with a hex wrench, but are subject to faster wear.

The material chosen for the ball or nose is also key: case hardened steel noses are strong and heat-resistant, but can mar the surface to which they press. Delrin® is a very strong plastic that holds up well, is self-lubricating, and will not mar. Nylon also resists marring and corrosion but doesn’t wear as well and can’t be used in temperatures above 82ºC (180ºF). Phenolic plastic is an economical option but is more brittle. Stainless steel is the best choice for sterile applications.

Body attributes: The body of a plunger varies depending upon installation considerations. It may be threaded, it may have a lock nut for securing the mounting (see illustration). Or it might be smooth for push-fit installation, or smooth and weldable. Threaded body plungers may have a locking element or patch. There may be a hex or slot on the end to assist in mounting.

The same material issues apply to the body as to the nose or ball. Case-hardened steel stands up to high forces. Steel may be zinc-plated or have a black oxide finish. Stainless steel tolerates high forces, heat, and corrosion. Brass is another option for metal plungers. As for plastic, a Delrin® body is non-magnetic and resists corrosion but will not tolerate heat.

Space considerations may require you use a short or stubby plunger. An extra-long plunger (“long-travel”) or long nose can be used to meet side force holding requirements or for holding up metal sheets. This latter is illustrated by our GN 611 long stroke spring plungers.

Springs: Springs are generally made of steel or stainless steel, and their end force determines the applications for which the plunger is appropriate. For detents, you will need a heavy or standard end force. If you want an easily retractable plunger, use standard or light end force. Heavy end forces permit the plunger to hold its position by end force alone. Light end forces prevent marring. Many of our plungers are available in all three versions.

Hand-operating elements: For hand-retractable plungers, the plunger incorporates an element that the hand can hold, pull, or manipulate. Options include knobs with or without knurling, L-handles, T-handles, and pull rings. Some plungers have a threaded end that will accept whatever knob or handle the user wishes to install. There is also the cam-action indexing plunger, which is designed so the turning of a handle in a circular direction causes the nose to retract.

J.W. Winco offers the unique type of cam-action indexing plunger pictured here, which permits the plunger pin to be held in either a retracted or protruding position. View GN 712 and GN 712.1 for full details.

Lock out or non-lock out: Last, but not least, is the issue of whether or not the plunger needs the option of holding a retracted position. Lock out type plungers are designed so the operator can pull the plunger into a retracted position, perform a turn of the knob or handle, and let go, and the plunger nose will stay retracted. Cam-action plungers may be either version.

Receptacles for plungers: Obviously the ball, nose, or pin of a plunger or release pin needs a receptacle. The receiving element need be no more complicated than a drilled hole or indentation. It may also be a flange or a mounting block that is mounted by welding or screws to a surface. See the illustration for an example using a mounting block, our GN 412.2.

Accessories: Spacer bushings are a useful component of plunger applications, and compensate for the body thread lengths on indexing plungers to allow mounting through walls of varying thicknesses. Ball chains and lanyards can be used as ways to “tie” removable plungers and release pins to a piece of equipment so they will not be lost, as you see in the illustration (our GN 113.3 rapid release pins with a ball chain).

Well, as far as these varied and useful positioning and locking components, that’s all I got. But if you want to know even more, visit Section 10 of our online catalog, or contact one of our Technical Sales associates!